The present embodiments relate to a magnetic resonance imaging (MRI) scanner having a device for compensating temperature fluctuations, and to a corresponding method.
MRI scanners are based on measuring the resonance behavior of nuclear spins in external magnetic fields and are therefore also known as nuclear magnetic resonance imaging scanners. The basic principle of MRI involves a static magnetic field (e.g., B0 field) for aligning the nuclear spins and adjusting the rotation frequency of the nuclear spins, and a time variable magnetic field (e.g., B1 magnetic field) for flipping the nuclear spins. In order to examine the resonance behavior of the nuclear spins, the reaction over time to transient deflections caused by the transiently variable magnetic fields (e.g., gradient fields) is also measured.
The time variable B1 magnetic field allows spatial assignment of the detected signals as the basis for three-dimensional acquisition and imaging. The time variable B1 magnetic field is produced by a B1 coil arrangement that generates a signal of constant frequency (e.g., 123.1 MHz). The correlation between B1 frequency and B0 magnetic field strength is critical for the spatial resolution in MRI scans, so that, to maximize the spatial resolution, the correlation is to be kept as constant as possible or is to be known as precisely as possible.
Gradient fields are generated by gradient coils that enable the desired magnetic fields and time characteristics to be produced with minimum distortion and maximum stability. Demanding MRI sequences (e.g., time characteristics of gradient fields) such as functional MRI (fMRI), fat-saturation imaging (FATSAT) or single voxel spectroscopy (SVS), for example, place extremely exacting requirements in terms of the non-distortion of the magnetic fields and the timing.
Magnetic fields may be distorted by, among other things, eddy currents that are induced in the magnet arrangements by time varying magnetic fields (e.g., gradient fields or gradient pulses). Time characteristics of magnetic fields (e.g., frequency and phase) may be skewed by, among other things, temperature fluctuations of the coils (e.g., gradient coils) and of other magnetically active components present in the magnetic field. Distortions of the magnetic fields (e.g., also of the static field) may also be caused by magnetically active bodies in the MRI scanner or in the vicinity of the MRI scanner (e.g., ferromagnetic metal bodies of the housing and of the functional components of the MRI scanner). Insofar as the causes of the distortions are static, the causes of the distortions may be compensated or reduced my mounting shim irons.
Distortions of the magnetic fields may have dynamic causes (e.g., changing magnetic fields). The publication “Rapid eddy current calibration and prospective distortion correction methods for diffusion-weighted MRI,” by M. Zaitsev, J. Hennig, and O. Speck, in Proc. Intl. Soc. Mag. Reson. Med. 13 (2005), discloses a method known as MESON for compensating distortions of magnetic fields caused by induced eddy currents in MRI.
Non-static distortions may be caused by unwanted temperature variations occurring during operation of the MRI scanner. Changes in the temperature of the scanner components cause the magnetic properties to change. As the components of a scanner generally heat up during operation, temperature-dependent magnetic field distortions are unavoidable, causing, for example, the static B0 magnetic field to become distorted. This results in changes in the strength of the B0 magnetic field that cause changes in the correlation between B0 field strength and B1 field frequency.
The publication US 2009/0140735 A1 therefore discloses an MRI scanner having a coil fitted with a heater and temperature sensor. The coil is heated to a controlled, constant temperature in order to eliminate temperature fluctuations. The need to continuously heat the coil adversely affects energy consumption, as the additional coils of the MRI scanner may be of the superconducting type and therefore are to be deeply cooled.
The publication US 2003/0164702 A1 discloses an MRI scanner that incorporates shim irons or shims that are configured to increase the homogeneity of the magnetic fields. The shim irons are disposed on the gradient coils. During operation of the scanner, the shim irons are subject to temperature fluctuations caused by magnetic fields and heating of the gradient coils. The magnetic properties of the shim irons are temperature-dependent. It is therefore proposed to keep the shim irons at constant temperature by controlled heating. As mentioned above, the heating device disadvantageously affects energy consumption.